Compressor wheels experience significant tensile stresses when rotated at high speeds, for example, compressor wheels used to assist internal combustion engine may be rotated at speeds in excess of 100,000 revolutions per minute. Further, fatigue can occur as rotational speed and other operating conditions vary. Fatigue may be defined as failure under a repeated or otherwise varying load, which does not reach a level sufficient to cause failure in a single application. Fatigue is known to be an issue for lightweight compressor wheels which are typically made from cast or forged aluminum alloy that may have impurities that are difficult to control. Fatigue is often associated with small scale cracks that develop in response to cyclic plastic deformations in a localized region. Such cracks usually originate at a preexisting, small scale defect at a material surface, which may be associated with impurities, manufacturing processes, etc. Consequently, surface quality can have a profound effect on crack initiation and hence fatigue.
To improve surface quality of a component, and thereby fatigue resistance, a variety of surface treatment techniques have been developed. Such techniques usually aim to induce a residual compressive stress at the surface of a material that can counter tensile stress associated with loading. One class of techniques are known as “cold working” techniques or processes. Cold working processes include shot peening and others. In shot peening, a surface is bombarded with small particles called shot that create dimples. Overlapping dimples develop a layer of residual compressive stress. Surface regions under compressive stress seldom initiate or propagate cracks. Shot peening can also increase surface hardness of a material.
Recently, cold working processes have been applied to conventional compressor wheels. U.S. Pat. No. 6,146,931 to Norton et al. discloses use of cold working processes to treat the bore of a conventional compressor wheel and thereby reduce failure caused by operational tensile loading. Norton et al. define the bore by an inner circumference that extends the entire length of the conventional compressor wheel. Their cold working process treats the entire inner circumference surface that defines the bore.
For a variety of reasons, “boreless” compressor wheels have been developed that include a joint that does not extend the entire length of the compressor wheel; thus, boreless compressor wheels cannot employ a shaft that extends the length of the wheel. Further, fatigue characteristics of a boreless compressor wheel differ from those of a conventional compressor wheel. Exemplary devices, methods, systems, etc. are presented below that address fatigue reduction in boreless compressor wheels.